Tracking System Using Field Mapping

ABSTRACT

In some aspects, a method includes (i) securing multiple sets of current injecting electrodes to an organ in a patient&#39;s body, (ii) causing current to flow among the multiple sets of current injecting electrodes to generate a field in the organ, (iii) in response to current flow caused by the multiple sets of current injecting electrodes, measuring the field at each of one or more additional electrodes, (iv) determining expected signal measurements of the field inside the organ using a pre-determined model of the field, and (v) determining a position of each of the one or more additional electrodes in the organ based on the measurements made by the additional electrodes and the determined expected signal measurements of the field.

TECHNICAL FIELD

This invention relates to determining the position of an object, such astracking the position of one or more catheters in a patient's heartcavity.

BACKGROUND

Use of minimally invasive procedures, such as catheter ablation, totreat a variety of heart conditions, such as supraventricular andventricular arrhythmias, is becoming increasingly more prevalent. Suchprocedures involve the mapping of electrical activity in the heart(e.g., based on cardiac signals), such as at various locations on theendocardium surface (“cardiac mapping”), to identify the site of originof the arrhythmia followed by a targeted ablation of the site. Toperform such cardiac mapping a catheter with one or more electrodes canbe inserted into the patient's heart chamber.

Under some circumstances, the location of the catheter in the heartchamber is determined using a tracking system. Catheter tracking is acore functionality of modem mapping systems that also include softwareand graphic user interface to project electrical data on 3D renderingsof cardiac chambers. Currently there are several tracking systemsavailable, some more useful and commonly used than others. Some systemsare based on the use of magnetic or electric fields from externalsources to sense and track the location of the catheter. Some are basedon the use of magnetic or electric fields sources mounted on the trackedcatheters.

SUMMARY

In some aspects, a method includes (i) causing current to flow amongmultiple sets of current injecting electrodes to generate a field in anorgan, (ii) obtaining the positions of one or more measuring electrodesused for measuring the field generated by the current injectingelectrodes, (iii) in response to the current flow, measuring the fieldat multiple locations in the organ using the one or more measuringelectrodes, (iv) modeling the field using the measurements of the fieldfrom the one or more measuring electrodes and the positions of the oneor more measuring electrodes, and (v) determining expected signalmeasurements of the field at additional locations within the organ usingthe model of the field.

Embodiments can include one or more of the following.

Modeling the field can include modeling the field based on physicalcharacteristics of the organ. Modeling the field based on physicalcharacteristics of fields can include using Laplace's equation. Modelingthe field based on physical characteristics can include using Poisson'sequation. Modeling the field based on physical characteristics caninclude modeling a homogeneous medium. Modeling the field based onphysical characteristics can include modeling an inhomogeneous medium.Modeling of the field can include using a function that correlates fieldmeasurements with position coordinates. The function can be adifferentiable function.

The current injecting electrodes can be mounted on one or more cathetersthat are placed inside the body. The current injecting electrodes caninclude one or more body-surface electrodes. The current injectingelectrodes can include both electrodes mounted on one or more cathetersthat are placed inside the body and body-surface electrodes. Theelectrodes mounted on one or more catheters can include one or moreelectrodes secured to the organ. The electrodes mounted on the one ormore catheters can include one or more electrodes that can be moved andpositioned at multiple locations in an organ.

Obtaining the position of the one or more measuring electrodes caninclude using a tracking system to track the position of the one or moremeasuring electrodes. The tracking system can be a system using amagnetic field for tracking. The tracking system can be a system usinginjected currents for tracking.

Measuring the field can include measuring potentials.

The additional locations within the organ can include positions withinthe organ where the field was not measured. The positions within theorgan where field was not measured can include positions that are morethan 5 mm away from positions where the field was measured. Thepositions within the organ where field was not measured can includepositions that are not lying between positions where the field wasmeasured.

The method can also include determining a position of one or moreadditional electrodes in the organ based on measurements made by theadditional electrodes and the determined expected signal measurements ofthe field. Determining a position of one or more additional electrodesin the field based on measurements made by the additional electrodes andthe determined expected signal measurements of the field can includesolving an optimization problem that minimizes collective differencebetween each of the measured signals and an estimate for each of therespective measured signals as a function of the position of themeasurement.

Determining expected signal measurements can include determiningexpected signal measurements using a non-interpolation basedcalculation.

Modeling the field based on physical characteristics can include usingthe measured signals to determine an electric potential distributionproduced by the current injecting electrodes on a surface enclosing theadditional positions. The determination of the electric potentialdistribution can be based on an assumption of a homogenous conductivitywithin the surface. Modeling the field based on physical characteristicscan be based on the electric potential distribution.

The current-injecting electrodes can operate at a frequency differentfrom the frequency of normal electrical activity in the organ.

Obtaining the positions of the one or more measuring electrodes caninclude tracking coordinates for the multiple positions of the measuredfield.

The measuring of the field at the multiple locations can include movinga catheter having one or more measuring electrodes to multiple locationswithin the organ, and using the measuring electrodes to measure thefield for each of the multiple locations of the catheter. The additionallocations can correspond to regions inside the organ not interrogated bythe movement of the catheter. The multiple sets of current-injectingelectrodes can include at least three sets of current injectingelectrodes and causing of the current flow can include causing currentto flow between each set of current injecting electrodes. The fieldmeasured in response to the current flow can include an electric signalfor each set of the current injecting electrodes for each of themultiple positions.

Modeling the field can include generating a field map.

The current injecting electrodes can be supported on one or morecatheters, and at least one of the catheters further can includemeasuring electrodes for measuring at least some of the measured field.

The method can also include displaying the determined location of themeasuring electrode relative to a surface of the organ.

The organ can be a patient's heart.

The field can be a scalar value field. The field can be a potentialfield. The field can be an impedance field.

In some aspects, a system includes a first catheter configured forinsertion into an organ in a patient's body and comprising one or moremeasuring electrodes and multiple sets of current injecting electrodes.The system also includes an electronic control system coupled to themultiple sets of current injecting electrodes and to the one or moremeasuring electrodes. The control system is configured to cause currentto flow among multiple sets of current injecting electrodes to generatea field in an organ and to measure the field and in response to thecurrent flow, measure the field at multiple locations in the organ usingthe one or more measuring electrodes. The system also includes atracking system configured to obtain the positions of one or moremeasuring electrodes used for measuring the field generated by thecurrent injecting electrodes. The system also includes a processingsystem coupled to the electronic system. The processing system isconfigured to model the field using the measurements of the field fromthe one or more measuring electrodes and the positions of the one ormore measuring electrodes and determine expected signal measurements ofthe field at additional locations within the organ using the model ofthe field.

Embodiments can include one or more of the following. The processingsystem can be further configured to model the field based on physicalcharacteristics of the organ. The processing system can be furtherconfigured to model the field using Laplace's equation. The processingsystem can be further configured to model the field using Poisson'sequation. The current injecting electrodes can be mounted on one or morecatheters that are placed inside the body. The current injectingelectrodes can include one or more body-surface electrodes. The currentinjecting electrodes can include both electrodes mounted on one or morecatheters that are placed inside the body and body-surface electrodes.The electrodes mounted on one or more catheters can include one or moreelectrodes secured to the organ. The electrodes mounted on the one ormore catheters can include one or more electrodes that can be moved andpositioned at multiple locations in an organ. The tracking system can bea system using a magnetic field for tracking. The tracking system can bea system using injected currents for tracking. The multiple sets ofcurrent-injecting electrodes can include at least three sets of currentinjecting electrodes.

In some aspects an method includes (i) securing multiple sets of currentinjecting electrodes to an organ in a patient's body, (ii) causingcurrent to flow among the multiple sets of current injecting electrodesto generate a field in the organ, (iii) in response to current flowcaused by the multiple sets of current injecting electrodes, measuringthe field at each of one or more additional electrodes, (iv) determiningexpected signal measurements of the field inside the organ using apre-determined model of the field, and (v) determining a position ofeach of the one or more additional electrodes in the organ based on themeasurements made by the additional electrodes and the determinedexpected signal measurements of the field.

Embodiments can include one or more of the following.

Determining the position of the one or more additional electrodes in theorgan based on measurements made by the additional electrodes and thedetermined expected signal measurements of the field can include solvingan optimization problem that minimizes a collective difference betweeneach of the measured signals and an estimate for each of the respectivemeasured signals as a function of the position of the measurement.

The estimate for each of the respective measured signals can include adifferentiable function.

The one or more additional electrodes can include one or more electrodesused for delivering ablation energy for ablating tissue of the organ.The one or more additional electrodes can include one or more electrodesused for measuring the electrical activity of the organ.

The method can also include generating the pre-determined model of thefield.

Generating the pre-determined model of the field can include causingcurrent to flow among the multiple sets of current injecting electrodesto generate a field in an organ, obtaining the positions of one or moremeasuring electrodes, in response to the current flow, measuring thefield at multiple locations in the organ using the one or more measuringelectrodes, and modeling the field using the measurements of the fieldmeasured by the one or more measuring electrodes and the positions ofthe one or more measuring electrodes.

Modeling the field can include modeling the field based on physicalcharacteristics. Modeling the field based on physical characteristicscan include using Laplace's equation. Modeling the field based onphysical characteristics can include using Poisson's equation. Modelingthe field based on physical characteristics can include modeling ahomogeneous medium. Modeling the field based on physical characteristicscan include modeling an inhomogeneous medium. Modeling of the field canalso include representing the model using a function that correlatesfield measurements with position coordinates.

The pre-determined model of the field can be a field map. The field mapcan be a function that correlates the expected signal measurements withposition coordinates within the organ. The function can be adifferentiable function.

Measuring the field can include measuring potentials.

The current-injecting electrodes can operate at a frequency differentfrom the frequency of normal electrical activity in the organ.

The organ can be a patient's heart.

In some aspects, a system includes multiple sets of current injectingelectrodes configured to be secured to an organ in a patient's body. Thesystem also includes one or more additional electrodes configured to bepositioned within the organ in the patient's body. The system alsoincludes an electronic control system coupled to the multiple sets ofcurrent injecting electrodes and the one or more additional electrodes.The electronic control system can be configured to cause current to flowamong the multiple sets of current injecting electrodes to generate afield in the organ. The electronic control system can also be configuredto, in response to current flow caused by the multiple sets of currentinjecting electrodes, measure the field at each of one or moreadditional electrodes. The system also includes a processing systemcoupled to the electronic system. The processing system can beconfigured to determine expected signal measurements of the field insidethe organ using a pre-determined model of the field and determine aposition of each of the one or more additional electrodes in the organbased on the measurements made by the additional electrodes and thedetermined expected signal measurements of the field.

Embodiments can include one or more of the following.

The processing system can be configured to solve an optimization problemthat minimizes a collective difference between each of the measuredsignals and an estimate for each of the respective measured signals as afunction of the position of the measurement. The one or more additionalelectrodes can be one or more electrodes used for delivering ablationenergy for ablating tissue of the organ. The one or more additionalelectrodes can be one or more electrodes used for measuring theelectrical activity of the organ. The processing system can be furtherconfigured to generate the pre-determined model of the field.

In some aspects, a method includes (i) securing at least three sets ofcurrent injecting electrodes to an organ in a patient's body, (ii)causing current to flow among the multiple sets of current injectingelectrodes to generate a field in the organ, (iii) using amulti-electrode array located on a multi-electrode array catheter in theorgan for tracking a position of the multi-electrode array catheterrelative to the current injecting electrodes, (iv) measuring the fieldgenerated by the current injecting electrodes in multiple locations inthe organ using the multi-electrode array, (v) modeling the field usingthe measurements and the positions, (vi) determining expected signalmeasurements of the field at additional locations within the organ basedon the model of the field, and (vii) determining a position of one ormore additional electrodes in the organ relative to the currentinjecting electrodes based on measurements made by the additionalelectrodes and the determined expected signal measurements of the field.

Embodiments can include one or more of the following.

The method can also include removing multi-electrode array catheter fromthe organ prior to determining the position of one or more additionalelectrodes in the organ. The one or more additional electrodes caninclude one or more electrodes mounted on one of more additionalcatheters. The one or more additional electrodes comprise one or moreelectrodes of the multi-electrode array.

Modeling the field based on physical characteristics can include usingLaplace's equation. Modeling the field can include modeling the fieldbased on physical characteristics. Modeling the field based on physicalcharacteristics can include using Poisson's equation. Modeling the fieldbased on physical characteristics can include modeling a homogeneousmedium. Modeling the field based on physical characteristics can includemodeling an inhomogeneous medium. Modeling of the field can also includerepresenting the model using a function that correlates fieldmeasurements with position coordinates.

The additional locations within the organ can be positions within theorgan where the field was not measured. The positions within the organwhere field was not measured can be positions that are more than 5 mmaway from positions where the field was measured. The positions withinthe organ where field was not measured can be positions that are notlying between positions where the field was measured. Determining aposition of one or more additional electrodes in the field based onmeasurements made by the additional electrodes and the determinedexpected signal measurements of the field can include solving anoptimization problem that minimizes collective difference between eachof the measured signals and an estimate for each of the respectivemeasured signals as a function of the position of the measurement.Determining expected signal measurements can include determiningexpected signal measurements using a non- interpolation basedcalculation.

Measuring of the field at the multiple locations can include moving acatheter having one or more measuring electrodes to multiple locationswithin the organ, and using the measuring electrodes to measure thefield for each of the multiple locations of the catheter.

The additional locations can correspond to regions inside the organ notinterrogated by the movement of the catheter.

The multiple sets of current-injecting electrodes can include at leastthree sets of current injecting electrodes, and wherein the causing ofthe current flow comprises causing current to flow between each set ofcurrent injecting electrodes, and wherein the field measured in responseto the current flow comprise an electric signal for each set of thecurrent injecting electrodes for each of the multiple positions.

Modeling the field can include generating a field map.

The method can also include displaying the determined location of themeasuring electrode relative to a surface of the organ.

In some aspects, a system can include at least three sets of currentinjecting electrodes configured to be secured to an organ in a patient'sbody. The system can also include a multi-electrode array cathetercomprising a multi-electrode array configured to be inserted in theorgan for tracking a position of the multi-electrode array catheterrelative to the current injecting electrodes. The system can alsoinclude an electronic control system coupled to at least three sets ofcurrent injecting electrodes and to the multi-electrode array catheter.The electronic control system can be configured to cause current to flowamong the multiple sets of current injecting electrodes to generate afield in the organ and measure the field generated by the currentinjecting electrodes in multiple locations in the organ using themulti-electrode array. The system can also include a processing systemcoupled to the electronic system. The processing system can beconfigured to model the field using the measurements and the positions,determine expected signal measurements of the field at additionallocations within the organ based on the model of the field, anddetermine a position of one or more additional electrodes in the organrelative to the current injecting electrodes based on measurements madeby the additional electrodes and the determined expected signalmeasurements of the field.

Embodiments can include one or more of the following.

The one or more additional electrodes can be one or more electrodesmounted on one of more additional catheters. The one or more additionalelectrodes can be one or more electrodes of the multi-electrode array.The multiple sets of current-injecting electrodes can include at leastthree sets of current injecting electrodes.

Embodiments of the system may also include devices, software,components, and/or systems to perform any features described above inconnection with the first method and/or described below in connectionwith the second method.

Embodiments of the methods and systems generally disclosed herein can beapplied to determining the position of any object within an organ in apatient's body such as the patient's heart, lungs, brain, or liver.

As used herein, the “position” of an object means information about oneor more of the 6 degrees of freedom that completely define the locationand orientation of a three- dimensional object in a three-dimensionalcoordinate system. For example, the position of the object can include:three independent values indicative of the coordinates of a point of theobject in a Cartesian coordinate system and three independent valuesindicative of the angles for the orientation of the object about each ofthe Cartesian axes; or any subset of such values.

As used herein, “heart cavity” means the heart and surrounding tissue.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. In case of conflict withdocuments incorporated herein by reference, the present documentcontrols.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary schematic diagram of an arrangement forpositioning current injection electrodes (CIE) and potential measuringelectrodes (PME) with respect to a patient's heart cavity.

FIG. 2 is a flow diagram of an exemplary embodiment for determining thepositions of PME.

FIG. 3 is a flow diagram of an exemplary embodiment for generating afield map.

FIGS. 4 and 5 are exemplary schematic diagrams of an arrangement forpositioning current injection electrodes (CIE) and potential measuringelectrodes (PME) with respect to a patient's heart cavity.

FIG. 6 is a flow diagram of an exemplary embodiment for generating afield map and using the field map to determine the positions of PME.

FIG. 7 is an exemplary schematic diagram of an arrangement forpositioning current injection electrodes (CIE), potential measuringelectrodes (PME), and a multi- electrode array (MEA) catheter withrespect to a patient's heart cavity.

FIG. 8 is a flow diagram of an exemplary embodiment for generating afield map and using the field map to determine the positions of PME.

FIG. 9 is a schematic diagram of an exemplary system.

FIG. 10 is a schematic diagram of an analog implementation of a signalgeneration module (SGM) and signal acquisition module (SAM) for anelectronics module coupled current emitting electrodes and potentialmeasuring electrodes.

FIG. 11 is a schematic diagram of a digital implementation of a signalgeneration module (SGM) and signal acquisition module (SAM) for anelectronics module coupled to current emitting electrodes and potentialmeasuring electrodes.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Embodiments disclosed herein include a method and system for determiningthe position of a catheter in a patient's heart cavity using apre-determined model of the field that provides expected signalmeasurements of the field at various locations within the heart cavity.

More particularly, the methods and systems described herein provide amethod for tracking electrodes mounted on catheters within and relativeto the cardiac cavity, including any number of chambers within thiscavity and the blood vessels surrounding it, but it can be used fortracking catheters in other body organs as well. Electrodes can bemounted on one or multiple catheters and by tracking these electrodesthe location of such catheters can be determined and the catheters canbe tracked. By knowing the physical characteristics of a catheter andthe position of the electrodes on it, it is possible to track specificportion of the catheter (e.g., the tip) or to determine the shape andthe orientation of the catheter (e.g., by using a spline fitting methodon the location of multiple electrodes of the same catheter). Electrodescan also be mounted on other devices that require tracking inside theheart cavity.

In some aspects, the tracking is accomplished by generating a multitudeof fields using current injecting electrodes (CIE) positioned andsecured in a stable location (e.g., coronary sinus, atrial appendage,apex) and using measurements of the same fields on electrodes mounted onother catheters to locate the electrodes. The purpose of the CIEs is toinject current into the heart cavity. For example, each CIE pair candefine a source and sink electrode, respectively, for injecting currentinto the heart cavity.

In general, in one aspect, a field mapping catheter that includes one ormore potential measuring electrodes (PME) that can measure the fields(e.g., measure potentials in the heart cavity in response to the currentprovided by the CIEs) and at the same time can be tracked by anindependent tracking system is used for generating a field map. Thefield map provides expected signal measurements of the field at variouslocations within the heart cavity. A field map is an example of apre-determined model of the field. Other methods for generating apre-determined model of the field exist and can be used. For example, apre-determined model can be generated based on a volumetric image of themedium (CT or MRI) and an analysis of the medium based on that image togenerate a physical model of the fields in the medium.

An independent tracking system can be any system for tracking cathetersinside body organs, such as systems based on electric or magneticsignals generated externally and detected by one or more trackingelements, affixed to a catheter, or systems based on electric ormagnetic signals generated internally from a catheter and detected byone or more sensors external to the body or internal to the body,affixed to other catheters

The model of the field is generated using the field measurements and thepositions measurements collected by the field mapping catheter. Themodel can be generated based on physical characteristics of the medium.Such a physical model can be determined, for example, by solvingLaplace's equation in a homogeneous medium representing the cardiacchamber. The model of the field associates the field measurements toeach location in space.

Once a field map is generated the independent tracking system can beturned off, any internal element of the system used to generate thefield map can be taken out of the body, and the field mapping cathetercan also be taken out of the body. However, the CIE used to generate thefields are left in their stable locations for subsequent use in trackingother electrodes. In some embodiments, removing the potential measuringelectrodes used to generate the field map can be advantageous when it isdesired to have fewer catheters inside the body organ for clinicalreasons, or when certain tracking fields interfere with otherinstruments in the operating room. Using the field map it is possible todetermine the location of any potential measuring electrodes (PME) thatcan measure the generated fields (e.g., the fields generated using thecurrent injecting electrodes) inside the volume covered by the fieldmap. The position of a tracked PME is determined by comparing themeasured field value and the modeled field values. The position in thefield map that holds a value matching the measurement of the tracked PMEis assigned as the location of that PME.

In some embodiments, potentials measured in response to the injectedcurrent (e.g., tracking signals) can be used to continuously monitor theposition of one or more catheters in the heart cavity, even as thecatheters are moved within the heart cavity.

In the above discussion and in the details that follow, the focus is ondetermining the position of one or more catheters in a heart cavity fordiagnosis and treatment of cardiac arrhythmias. However, this is only anexemplary application. The method and system generally disclosed hereincould be used to track essentially any catheter mounted with at leastone electrode, regardless of the catheter's intended function. Relevantexamples include endocardial biopsies, therapies involvingintra-myocardial injections of cells, drugs, or growth factors, and thepercutaneous placement cardiac valves. In other examples, the method andsystems generally disclosed herein can be applied to determining theposition of any object within any distribution of materialscharacterized by a conductivity profile. For example, the methods andsystems generally disclosed herein can be applied to determining theposition of any object within an organ in a patient's body such as thepatient's heart, lungs, brain, or liver.

Furthermore, while in some of the specific embodiments that follow thesignals measured by the object electrodes correspond to the relativestrength (e.g., amplitude) of the measured electrical signal (e.g.,potential), further embodiments may also analyze the phase of themeasured signal, either alone or in combination with the amplitude ofthe measured signal. The phase of the measured signal is indicative ofspatial variations in the imaginary part of the complex conductivity(e.g., permittivity) in the distribution of materials.

In some aspects, the system tracks electrodes inside a body withouthaving these electrodes injecting currents or emitting any field thatneeds to be detected. Rather, the fields can be generated by CIEpositioned at fixed locations relative to the organ. This allowstracking of a large number of electrodes simultaneously, as the trackedelectrodes are not polled one after the other as is the case with someother tracking methods.

In some additional aspects, the system does not require any externalpatches to be attached to the body, or any other external energyemitter. In some embodiments, the system only uses internal electrodesto inject currents. Furthermore, the method does not require anyknowledge about the location in space of the current injectingelectrodes.

In some embodiments, the CIE can be positioned such that the currentinjection is taking place from objects that are secured to the heartitself, reducing inaccuracies from motion artifacts that are experiencedby systems that are referenced to an external coordinate system.

An inverse Laplace method is used to map the fields generated by theinjected currents. An inverse Laplace method is more accurate than othermethods used for volumetric field mapping (e.g., interpolation ofmeasured values over the volume of interest). The field map generated byusing the inverse Laplace method is more accurate in areas that were notprobed specifically by the field mapping catheter and in areas not lyingbetween positions that were probed.

In general, a field map generated by physical modeling of the medium,such as by using the inverse Laplace method, can be represented by adifferentiable function. Finding a match between a measurement and thefield map requires finding a minimum in the field using optimization.Optimization techniques of differentiable functions are believed to befaster and more accurate than other techniques.

FIG. 1 shows an exemplary schematic diagram of an arrangement forpositioning current injection electrodes (CIE) and potential measuringelectrodes (PME) with respect to a patient's heart cavity. Three CIEpairs (e.g., CIE₁₊-CIE¹⁻; CIE₂₊-CIE²⁻; and CIE₃₊-CIE³⁻) are on a singlecatheter positioned and secured in a stable location in the coronarysinus, outside of any heart chamber. As described herein, while shown aspositioned in the coronary sinus, other locations outside of the heartchamber, within the organ itself, and/or outside of the patient's bodycould be used to secure the CIE pairs.

A PME is mounted on another catheter that is placed within the cardiacchamber and can move relative to the cardiac chamber. The PME is able tomeasure the fields generated by the different CIE pairs. The catheterthat supports the PME can be tracked by an independent tracking systemand thus generate a model of the field, e.g., a field map, as describedbelow. More particularly, the PME measures the fields generated by theCIE while at the same time being tracked by an independent trackingsystem. The measured fields and determined locations (e.g., from theindependent tracking systems) are used to generate a model of the field,e.g., a field map, assigning field measurements to each location inspace.

Once a model of the field is available it can be used for determiningthe position of any catheter capable of measuring the generated fields.Using the field map it is possible to determine the location ofpotential measuring electrodes (PME) that can measure the generatedfields inside the volume covered by the field map. The position of atracked PME is determined by comparing the measured field value and themodeled field values. The position in the field map that holds a valuematching the measurement of the tracked PME is assigned as the locationof that PME.

Tracking by Fields

Referring to FIG. 2, a process 200 for tracking electrodes (e.g., PME)using a pre-determined model of the field such as a field map is shown.In step 204, a number of current injecting electrodes (CIE), mounted onone or more catheters, are positioned in a stable position in the heart.The CIE are secured in a way that does not allow relative movementbetween the electrodes and the heart walls. For example, the CIE can besecured either by choosing a location such that the catheter willconform to the anatomy and will stay in a fixed position (e.g., coronarysinus or apex), or by using a fixation mechanism (e.g., anchoringmechanisms or a balloon mechanism). Information about the exact locationof the catheter and the CIE is not necessary for tracking othercatheters based on current injected by the CIE.

It should be noted that a minimum of three CIE configurations arenecessary to span a three dimensional space and provide XYZ coordinatesof other electrodes. An example of a CIE configuration is a pair of CIEconfigured as a dipole, having one CIE acting as a current source andthe other CIE acting as a current sink. An electrode can be used in morethan one CIE configuration. The electrodes cannot all be in the sameplane or otherwise a 3D space cannot be spanned. For this reason aminimum of four CIE are needed.

In step 206, current is injected using the CIE configurations. In orderto inject current an electrode must have impedance that is low enough.Low impedance can be achieved by a sufficient surface area or by usingmaterials or coatings that lower the impedance of the electrode. Any lowimpedance electrode can be used for current injection and in a casewhere many or all electrodes on a certain catheter are capable ofinjecting current the designation of such electrodes as CIE on thecatheter only indicates that these electrodes are actually being usedfor current injection.

Knowledge of the spatial configuration of CIE is not required for thetracking system to operate as long as the pairs used for injecting thecurrents are spanning the three dimensional space. Additionally, theproperties of the medium and the inhomogeneity of the medium are notmodeled and no prior knowledge is required about the medium.

It should be further appreciated that other configurations of CIE arepossible as long as these configurations span the space. Examples ofsuch a configuration could be quadruples involving 4 CIE, or even anon-symmetrical configuration involving 3 CIE. CIE electrodes can be onthe same catheter or on different catheters. They can be in the samechamber, in different chambers or also in the cardiovascular systemsurrounding the heart. For simplicity the method using electrode pairswill be explained, but the same method can be applied using otherconfigurations. In such cases there is still a need for at least threeseparate configurations in order to span the three-dimensional space andprovide XYZ coordinates of other electrodes.

In step 208, potential measuring electrodes (PME) mounted on trackedcatheters measure fields (e.g., potentials) emanating from cardiacactivation, as well as the fields (e.g., potentials) generated by theCIE. A field can be any scalar field that associates a scalar value toevery point in space. The PME can measure different kinds of fields,such as electrical potential field such as the potential in every pointin space relative to a reference potential, an impedance field such asthe impedance difference between every point in space to a referencepoint, etc. There is a need to distinguish between the cardiacactivation signal and the signals from the CIE in order to separate thetracking signal being used for the location determination from thecardiac signal being used for generating the electrical activation maps.The CIE inject the current at a frequency higher than cardiac activation(cardiac activation <2 kHz, CIE>4 kHz, e.g., 5 kHz, 10 kHz, 25 kHz) suchthat the two types of signals can be distinguished using frequencyanalysis. It should be noted that other methods for distinguishingbetween the CIE signal and the cardiac activation signal can be used,such as injecting a spread-spectrum signal having a low energy level inthe frequency range of the cardiac activation signal, and detecting thisspread-spectrum signal in the signal collected by the all PME.

In order to span the space three separate configurations of CIE need toinject current (e.g., 3 pairs of CIE not residing in the same plane).There is a need to determine the source of the injected signal and totrace it to a specific CIE configuration. The three pairs of CIE injectthe current sequentially, one pair at a time, so that it is possible totrace the source of the measured PME signals to a specific pair. This iscalled time division multiplexing. In the case of time divisionmultiplexing, CIE are activated in sequence such that at one point intime one pair is activated (e.g., CEI₁₊ and CEI¹⁻) and at the next pointin time another pair is activated (e.g., CIE₂₊ and CIE²⁻). The switchingbetween pairs may occur every cycle (e.g., ⅕ kHz=200 μs) or every fewcycles (e.g., 20 cycles, 20×200 μs=4 mS). It should be noted thatfrequency or code division (spread spectrum) multiplexing, rather thantime division may be used to separate the signals. In the case offrequency multiplexing all CIE pairs may inject the current at the sametime, but each pair uses a different signal frequency. The signalcollected at the PME is filtered according to the frequency, and thesignal measured in each frequency is then associated with theappropriate originating pair.

In step 202, the system obtains a field map generated prior to thetracking of the PMEs. In general, the field map provides an expectedfield for a given location within the organ. In step 210, the trackingof the PME on the catheters is performed by solving an optimizationproblem that compares the measurement collected by the PME as a resultof activation of the CIE pairs, to expected measurements in a givenlocation provided in the field map. The location that minimizes thedifference between the expected field and the measured field is assignedas electrode location. Exemplary methods for obtaining the field map aredescribed in further detail below.

While in some of the specific embodiments that follow the signalsmeasured by the object electrodes correspond to the relative strength(i.e., amplitude) of the measured electrical signal (e.g., potential),further embodiments may also analyze the phase of the measured signal,either alone or in combination with the amplitude of the measuredsignal. The phase of the measured signal is indicative of spatialvariations in the imaginary part of the complex conductivity (e.g.,permittivity) in the distribution of materials.

The field mapping process explained below provides a field map, M,assigning field measurements to each location in space. In case of threeCIE pairs each location in space, ρ=(x,y,z) being the location in 3DCartesian coordinates, is assigned three measurements, (V₁, V₂, V₃),corresponding to the three different fields generated by the three CIEpairs. The field map can be represented as a function M(x,y,z) =(V_(E1),V_(E2), V_(E3)), when V_(E) stands for the expected voltage based on thefield map.

Correspondingly, three measured results (V_(M1), V_(M2), V_(M3)) arealso obtained from the tracked PME. The specific location will becomputed such that ρ minimize the expression:

$\begin{matrix}{\,_{\rho}^{\min}{{\sum\limits_{i = 1}^{3}\; \left( {V_{Mi} - V_{Ei}} \right)^{2}}}} & (1)\end{matrix}$

Equation (1) is a non-linear optimization problem. This problem issolved using an iterative scheme such as Newton-Raphson orLevenberg-Marquardt or a direct search method such as the Nelder-MeadSimplex Method.

In the case of more than three pairs of CIE the solution for ρ becomesoverdetermined since we obtain more equations than unknowns, which helpsimprove tracking accuracy depending on the specific embodiment.

This method determines the location of PME without any prior knowledgeof the CIE spatial configuration or any prior knowledge of thecharacteristics of the medium.

In some embodiments, more than one PME may be tracked simultaneouslyusing process 200. To do so, signals are acquired from and anoptimization problem is solved for each of the electrodes being tracked.If such electrodes are mounted on different catheters, it is possible tosimultaneously track multiple catheters.

Systems that require the tracked electrode to inject current usuallytrack a single electrode at any given time, and for the tracking ofmultiple electrodes such systems usually activate one electrode at atime and sequentially cycle through all tracked electrodes. Since thereis a minimum duration that each electrode needs to be active in such asystem, and there is also a desired refreshing rate for the trackedlocation, there is a limit to the number of electrodes that can betracked simultaneously in such systems. Due to the passive nature of thetracked PME in the disclosed invention there is no limit to the numberof PME that can be tracked simultaneously.

As noted above, the measurements collected at the PMEs as a result ofcurrent injected by the CIE are generally affected by the complexconductivity, or admittivity, distribution of the medium. While thespecific embodiment discussed above focus on the real part of theconductivity which affects the amplitude measured by the PMEs,additional information can also be obtained by accounting for the realpart (conductivity) and imaginary part (permittivity) of the medium'scomplex conductivity, which affects the amplitude and phase of thesignal measured by the PME. In this manner, the use of both amplitudeand phase, or phase alone may also be used for tracking purposes. Use ofthe imaginary part of the complex conductivity is of particularimportance in material distributions where the permittivity contrastexceeds that of the conductivity contrast.

To modify the mathematical formalism for the specific embodimentsdescribed above to account for imaginary part of the complexconductivity each location in space, ρ=(x,y,z) being the location in 3DCartesian coordinates, is assigned three complex measurements, (V₁*,V₂*, V₂*). The fields map can be represented as a complex functionM(x,y,z)=(V*_(E1), V*_(E2), V*_(E3)).

Correspondingly, three complex measured results (V*_(M1), V*_(M2),V*_(M3)) are also obtained from the tracked PME. The specific locationwill be computed such that ρ minimize the expression:

$\begin{matrix}{\,_{\rho}^{\min}{{\sum\limits_{i = 1}^{3}\; \left( {V_{Mi}^{*} - V_{Ei}^{*}} \right)^{2}}}} & (2)\end{matrix}$

Equation (2) can be solved using the similar methods used for solvingequation (1).

Generating the Field Map

As described above, the field map is a function, M, which correlatesscalar measurements of the fields generated by the different CIE setswith position coordinates of an independent tracking system. Anindependent tracking system is one of any tracking systems that existfor tracking catheters inside body organs. Such a system can be based,for example, on tracking electric or magnetic signals generatedexternally and detected by one or more tracking elements, such assensors, affixed to a catheter. Alternatively, tracking elements such asemitters or beacons affixed to the catheter may emit electric ormagnetic signatures that are detected by an independent tracking system,and used to determine the location of the emitters, and thus thelocation and orientation of a catheter. For example, a collection ofminiaturized coils oriented to detect orthogonal magnetic fields andforming a sensor can be placed inside the catheter to detect thegenerated magnetic fields. The independent tracking systems are oftendisposed outside the patient's body at a distance that enables thesystem to either generate radiation of suitable strength (i.e., generatesignals whose amplitude will not harm the patient or otherwise interferewith the operation of other apparatus disposed in the near vicinity ofthe sensing and tracking system), or detect magnetic or electricradiation emitted by the emitters affixed to a catheter.

Pending patent application Ser. No. 12/061,297 entitled “IntracardiacTracking System” and filed Apr. 2, 2008, whose disclosure isincorporated herein in its entirety by reference, describes analternative independent tracking system utilizing a multi-electrodearray (MEA) for generating and sensing fields in the cavity for trackingPME and catheters. The system utilizes reference electrodes secured instable positions to reference the tracking coordinate system to theorgan, compensating for movement of the cavity in space that can resultfrom different reasons such as patient movements or patient respiration.

In case the independent tracking system generates fields in the mediumby injecting currents there is a need to determine the source of themeasured field signal and to trace it to a specific origin. It ispossible to use separate frequencies for the two systems, for exampleuse 5 KHz for the independent tracking system and 12 KHz for thecurrents injected by the CIE of the disclosed invention. The signalcollected at the PME is filtered according to the frequency, and thesignal measured in each frequency is then associated with theappropriate originating system. It is also possible to inject thecurrent sequentially, one set at a time, cycling through the currentinjecting sets of the independent tracking system and the CIE sets ofthe disclosed invention, so that it is possible to trace the source ofthe measured PME signals to a specific source. This way the timedivision multiplexing is used for both systems in a synchronous way. Itshould be noted that spread spectrum can also be used for the CIE of thedisclosed invention to prevent interference with the independenttracking system while keeping the ability to detect the injected fieldand to measure it.

The field mapping process uses a catheter that can be tracked by theindependent tracking system and that has at least one PME that canmeasure the fields generated by the CIE. The MEA catheter described inpending patent application Ser. No. 12/061,297 entitled “IntracardiacTracking System” and filed Apr. 2, 2008 is an example of a catheter thatcan be used for the field mapping process. The catheter used can bereferred to as the field mapping catheter.

Referring to FIG. 3, a process for generating a field map is shown. Instep 232, the system collects location data and field measurements atfield mapping points. More particularly, the field mapping catheter ismoved around inside the organ of interest while being tracked by theindependent tracking system and while measuring the generated fields ofthe CIE that are secured in fixed locations relative to the organ. Whilethe example shown in FIGS. 1, 4, and 5 includes three CIE pairs on asingle catheter positioned outside of any heart chamber variouslocations (e.g., other locations outside of the heart chamber, withinthe organ itself, and/or outside of the patient's body) could be used tosecure the CIE pairs.

The processing unit in the tracking system records the location in spaceof the field mapping catheter and stores for each location the fieldmeasurements measured by the PME of the field mapping catheter. Forexample, in the case of three CIE pairs each location in space,ρ=(x,y,z) being the location in space in the coordinate system of theindependent tracking system, is assigned three measurements, (V₁, V₂,V₃). The processing unit stores a table holding the different locationsand the different measurements T(x_(i),y_(i),z_(i))=(V_(i1), V_(i2),V_(i3)), i being the index number of the stored location. The locationsin which the fields were sampled and the results were stored in thetable T are referred to as field mapping points (FMP).

Cardiac contraction changes the medium in which the fields are beinggenerated thus changing generated fields. For that reason the fieldmeasurements are gated according to the cardiac cycle. This can be doneby using electrical measurements of the cardiac cycle (e.g., by the useof surface ECG) and triggering on a constant marker in the cardiac phase(e.g., using an R-wave detection algorithm, a threshold criterion, or amaximum criterion). Another option is to use a measurement that isaffected directly by the mechanical movement of the heart, such as themeasurement of the impedance between CIE, which changes as the distancebetween them changes as the heart contracts, and triggering on aconstant marker in the cycle. Once a trigger is determined the cardiaccycle is divided into m slices (e.g., m=10), and the mentioned recordingis repeated for each slice separately. This method results in m separatetables, T_(m)(x_(i,m),y_(i,m),z_(i,m))=(V_(i1,m,) V_(i2,m), V_(i3,m)),each one should be used for the appropriate phase of the heart cycle.

The table holding the locations and the measurements, T, is used forgenerating the field map M. Field map M is a function providing fieldsmeasurements for each location in space, for example M(x,y,z)=(V_(E1),V_(E2), V_(E3)) in the case of three CIE pairs. The function needs toprovide field values with spatial resolution corresponding to therequired accuracy of the tracking system.

One way to generate function, M, from the data table, T, is to generatea 3D grid, G, with a resolution that fits the required accuracy of thetracking system and use interpolation techniques on the values in tableT. An interpolation algorithm, such as cubic interpolation, is used tointerpolate the values stored in T onto G. The function M is defined asthe interpolated values on the grid G. When using the interpolationmethod for generating the field map function the resolution of thetracking system is limited by the resolution of the interpolation gridand by the accuracy of the interpolation itself. Furthermore, thefunction M generated in this method is not differentiable by definition,and therefore not all schemes mentioned for solving the minimizationproblem above are applicable. Solving the minimization problem fordetermining the location is slower in this case. Another disadvantage ofthis method is low accuracy in regions that have a large distancebetween FMP. For example, FIG. 4 shows an exemplary schematic diagram ofan arrangement for positioning current injection electrodes (CIE) andpotential measuring electrodes (PME) with respect to a patient's heartcavity. While the example shown in FIG. 4, includes three CIE pairs on asingle catheter positioned outside of any heart chamber other locationsoutside of the heart chamber, within the organ itself, and/or outside ofthe patient's body could be used to secure the CIE pairs. The schematicdiagram of FIG. 4 also shows multiple field mapping points (FMP) withrespect to a patient's heart cavity. In the example shown in FIG. 4, thearea marked “A” exhibits a large distance between FMP and therefore theaccuracy determined using an extrapolation/interpolation technique wouldbe lower in area A. Additionally, areas that are not lying between FMP(such as the area marked “B” in FIG. 4) require extrapolation, which haslower accuracy as the distance from the FMP increases.

As described above, using extrapolation/interpolation to generate thefunction M can result in a lower accuracy for points that are notlocated near the FMP. Rather than using extrapolation/interpolation, thefield map function, M, can be generated by modeling the field. Modelingthe field can provide a powerful tool for tracking the location ofelectrodes within an organ because the model of the field can provide ahigher accuracy in the expected signal measurements than using anextrapolation/interpolation technique to generate the function. Modelingthe field uses the measurements of the field from measuring electrodesand the positions of the measuring electrodes to determine expectedsignal measurements of the field at additional locations within theorgan. Modeling the field can use a mathematical calculation, such asLaplace's equation or Poisson's equation, that takes into account thephysical characteristics of organ, for example by modeling a portion ofthe organ as a homogeneous medium. Unlike an extrapolation/interpolationbased technique, modeling the field generates a function that isdifferentiable. Since finding a match between a measurement and thefield map includes finding a minimum in the field using an optimization,providing a differentiable function for the field map is believed toenable the user of faster and/or more accurate optimization techniques.

An example of a method of generating the field map function, M, bymodeling the field is the use of inverse theory. FIG. 5 shows anexemplary schematic diagram of multiple field mapping points (FMP) and adefined closed surface, S, encompassing the FMP but not enclosing theCIE. Referring back to FIG. 3, as described above in relation to step232, the potential field generated as a result of the injected currentfrom the CIE can be the field measured by the PME in the blood volume ofthe cavity. In step 234, a closed surface, S, is defined inside theblood volume having no CIE in it. Because blood is a homogenous mediumand the volume inside that closed surface has no sources (e.g., no CIE),the CIE can be modeled as a voltage distribution outside that closedvolume. This voltage distribution gives rise to an unknown Dirichletboundary condition, or voltage distribution, on the closed surface. Instep 236, the voltage distribution Vs on the surface S can becalculated, for example by solving an inverse Laplace problem asdescribed below.

In another example, if the CIE are positioned outside of the cardiacchamber, it is possible to use the surface of the chamber's anatomy asthe closed surface having the Dirichlet boundary condition. The anatomyof the chamber can be generated using the independent tracking system atthe time of the field map generation. In some embodiments, the anatomycan be generated using a point cloud method, such as the methoddisclosed, for example, in U.S. Pat. No. 6,226,542 the contents of whichis incorporated by reference herein. Another option to obtain theanatomy is registering a segmented imaging modality (e.g., a CT scan ofthe organ) to the coordinate system of the independent tracking system.An example of the use of an imaging modality is disclosed, for example,in patent application Ser. No. 11/451,898, entitled “NON-CONTACT CARDIACMAPPING, INCLUDING MOVING CATHETER AND MULTI-BEAT INTEGRATION” and filedJun. 13, 2006, the contents of which is incorporated by referenceherein. When generating the anatomy it is desired to use the phase ofthe cardiac cycle that will be later used for tracking. This can be donesimilar to the ways mentioned above for accounting for the contractioncycle when collecting the field mapping points using PME. It is alsopossible to use several anatomical representations, one corresponding toeach phase of the cardiac cycle, m, and use the multiple representationsfor generating different field maps, one for each phase of the cardiaccycle.

In general, the surface, S, can be modeled as having a voltagedistribution Vs representing the field sources. The field inside thevolume contained by surface S will follow Laplace's equation. In otherwords, the measurements collected by PME can be treated as a propagationof the voltage distribution Vs from the surface S to the PME whichfollows Laplace's equation. It follows that the voltage distribution Vscan be computed using an inverse Laplace algorithm based on measurementscollected by PME in the field mapping process and stored in table T. Inthis manner the field in the entire volume can be calculated from theboundary condition Vs generated by the inverse Laplace algorithm on thedata stored in table T. Referring again to FIG. 3, in step 238, a fieldmap function M is generated as the forward operator from the surfacedistribution Vs onto any point inside the volume enclosed by thesurface. An exemplary a method to compute all the fields at any locationinside the closed surface (e.g., a method to compute field map functionM) using inverse theory is described below. It should be noted that toperform the computation a surface S is constructed that is contained inthe blood volume and does not contain any CIE.

For each field generated by a CIE set the inverse procedure is doneseparately, and the process is repeated for all CIE sets (for examplethree times if the minimum of 3 CIE sets are used).

The physical laws governing the reconstruction of the fieldrepresentation on at the surface S are briefly summarized below:

The potential V in a homogeneous volume Ω is governed by Laplace'sequation

∇² V=0   (3)

subject to boundary conditions

V=V_(s) on surface S   (4)

where S represents the surface for solving the boundary condition.

Numerical methods such as boundary element method (BEM), finite elementmethod (FEM), finite volume method, etc. may be used to solve Laplace'sequation. Each numerical method represents the geometry and signal usingbasis functions, but each method uses its own representation. In allnumerical methods the potentials on the surface and on the field mappingpoints (FMP) are represented by finite-dimensional vectors. SinceLaplace's equations are linear, these vectors are related by a matrix A,known as the forward matrix:

V _(FMP1) =A·V _(S1)   (5)

V_(FMP1) is the vector containing the measurements of the fieldgenerated by the first set of CIE in the field mapping points (FMP). Thevector has the dimension n×1 where n is the number of FMP that wererecorded during the field mapping process. V_(S1) is a vector containingthe voltage distribution on surface S while the first CIE set is active.

The matrix A has dimensions of n×m, where n is the number of FMPlocations and m is the number of degrees of freedom in the surfacepotential, usually the number of surface elements used to represent thesurface S.

It is important to note that the 3D coordinates of FMP are used for theconstruction of A.

Equation 5 provides a forward relationship between surface voltageV_(S1) and the FMP voltages V_(FMP1). In the field mapping problemsurface voltage V_(S) is unknown while the measured FMP voltagesV_(FMP1) are known. An inverse relationship is employed to solve forV_(S). This inverse relationship may be posed as a least squaresoptimization problem:

$\begin{matrix}{\min\limits_{{\hat{V}}_{S}}\left( {{{{A \cdot {\hat{V}}_{S\; 1}} - V_{{FMP}\; 1}}}^{2} + {\alpha \cdot {{L \cdot {\hat{V}}_{S\; 1}}}^{2}}} \right)} & (6)\end{matrix}$

Where V_(FMP1) are measured potentials, A is the forward operator asdefined in equation 5, α is a regularization parameter, L is aregularization operator, and {circumflex over (V)}_(S1) is the vectorrepresenting the unknown surface voltage that is being calculated.Examples of the use of inverse theory and regularization are described,for example, in patent application Ser. No. 11/451,898, entitled“NON-CONTACT CARDIAC MAPPING, INCLUDING MOVING CATHETER AND MULTI-BEATINTEGRATION” and filed Jun. 13, 2006, the contents of which isincorporated by reference herein.

Tikhonov regularization may be used in this case. In the case ofTikhonov 0 regularization operator L is the identity matrix, while inthe case of Tikhonov 1 L is the gradient operator on surface S. In someexamples, Tikhonov 1 outperforms Tikhonov 0 and a regularizationparameter α=0.1 is found to be effective.

It should be noted that in order to determine {circumflex over (V)}_(S1)there is no need to move the field mapping catheter through the entirevolume contained inside the surface S. Further, the density of FMP doesnot have to be the same as the required resolution of the trackingsystem. It is preferred to have FMP points close (e.g., about 5 mm) tothe surface area that is closest to the CIE in order to model the CIEeffect on the surface S with high spatial resolution. The inverse theoryprojects the available measurements on the surface and allows computingthe field anywhere inside the enclosed volume.

With {circumflex over (V)}_(S1) known, it is possible to compute theexpected voltage measurement anywhere inside surface S in a manneridentical to equation 5, except that it is done for a particularlocation of interest.

The process is repeated for all generated fields. This results inseveral separate boundary conditions {circumflex over (V)}_(Sj). In thecase of three CIE pairs, for example, j=3. In this example the field mapfunction M is defined as M(x,y,z)=(A_(x,y,z)·{circumflex over (V)}_(S1),A_(x,y,z) {circumflex over (V)}_(S2), A_(x,y,z)·{circumflex over(V)}_(S3)) where A_(x,y,z) is defined as the forward matrix forcalculating the field in location x,y,z from the boundary distributionon the surface S.

This method generates a field map function M which is accurate for theentire enclosed volume using inverse Laplace theory.

Similar methods can be used for generating field maps for differentkinds of scalar fields. An impedance field can be generated using thesame inverse approach to achieve an accurate and differentiableimpedance field map without interpolation. In the case where there areelectrodes injecting currents inside the volume, such as the case inwhich the field mapping catheter is involved in the current injection, asimilar inverse method can be used. In such a case instead of usingLaplace's equation a more general representation of the electricaldistribution, called Poisson's equation, can be used. Similar tools canbe used for solving the inverse Poisson problem and generating a fieldmap.

Referring to FIG. 6, in some examples generating the field map andtracking by fields using the field maps, can be combined and executed ina single sequence on a patient using multiple catheters. Process 250describes a method for determining the position of PME within an organusing a field map in which the catheter used to generate the field mapcan be removed prior to tracking the location of the PME. For example,PME on one catheter can be used to generate the field map which is usedto track PME on another catheter such as an ablation catheter. In step251, current injecting electrodes are positioned in the cavity. The CIEare used both for providing the signals used in generating the field mapand for providing the signals used in tracking the locations of PMEusing the field map. In step 252, a catheter for field mapping isinserted into the cavity. In step 253, current is injected using theCIE. In step 254, the field mapping catheter is tracked by anindependent tracking system. In step 256, location data and fieldmeasurements are collected at multiple field mapping points within thecavity. In step 258 a closed surface, S, inside the volume and notincluding the CIE is generated. In step 260, the voltage distribution onthe surface is calculated. Using the calculated voltage distribution, instep 262, a field map is defined as the forward operator from thesurface to any point inside the volume enclosed by the surface. Once asufficient field map is generated, in step 264, the independent trackingsystem can be turned off, any internal element of that system can betaken out of the body, and the field mapping catheter can also be takenout of the body. This is advantageous when it is desired to have fewercatheters inside the body organ for clinical reasons, or when certaintracking fields interfere with other fields. In step 270, the voltageson the tracked PME are measured. The fields measured by the PME aregenerated using the CIE (e.g., as described in step 268). In step 272,the position of the PME is determined by solving an optimization problemusing the previously generated field map.

As noted above, in some embodiments, a multi-electrode array can be usedto determine the location of the electrodes within the organ whilecollecting measurements of the fields to generate the field mappingpoints. FIG. 7 shows an exemplary schematic diagram of an arrangementfor positioning current injection electrodes (CIE), a multi-electrodearray (MEA) catheter, and potential measuring electrodes (PME) withrespect to a patient's heart cavity.

Three CIE pairs (e.g., CIE1+-CIE1−; CIE2+-CIE2−; and CIE3+-CIE3−) are ona single catheter positioned and secured in a stable location in thecoronary sinus, outside of any heart chamber. The placement of the CIEpairs in the coronary sinus provides a fixed location for the CIE pairs.As described herein, while shown as positioned in the coronary sinus,other locations outside of the heart chamber, within the organ itself,and/or outside of the patient's body could be used to secure the CIEpairs.

Using an MEA catheter provides an alternative to using an independenttracking system for tracking PME and catheters while generating thefield mapping points. An exemplary MEA catheter is described in Pendingpatent application Ser. No. 12/061,297, entitled “Intracardiac TrackingSystem” and filed Apr. 2, 2008 whose disclosure is incorporated hereinin its entirety by reference. The exemplary MEA catheter includes CIEfor the purpose of independently tracking its position and the positionsof other PME and catheters. The same MEA catheter can also be used forthe field mapping process. In general, in the description below, threecatheters are used: (1) A MEA catheter is used for generating the fieldmap (2) another catheter (referred to as the tracked catheter) istracked based on the generated field map (3) a catheter that includesCIE is secured to secured to a stable position. For simplicity, the CIEmounted on the MEA catheter will be referred to as “MCIE” while the CIEof the current invention, that are secured to a stable position and thatare used for field mapping and for tracking of PME based on that map,will be referred to as the “SCIE.” For simplicity, the PME mounted onthe MEA catheter will be referred to as “MPME” and the PME mounted onthe tracked catheter will be referred to as “TPME.” While the exampledescribed above uses three catheters (e.g., the MEA catheter, thetracked catheter, and the secured catheter), in some embodiments, theSCIE can be mounted on multiple different catheters.

The MEA catheter can measure fields generated by the SCIE whiledetermining the location of the catheter within the organ. The measuredfields and determined locations are used to generate a field mapassigning field measurements to each location in space. After the fieldmapping process is complete, the MEA catheter can be removed from theorgan.

The tracked catheter that includes the TPME (a catheter other than theMEA catheter) is placed within the cardiac chamber and can move relativeto the cardiac chamber. The TPME is able to measure the fields generatedby the SCIE. The catheter can be tracked using the field map availablefor the chamber in which the catheter is positioned. Using the field mapit is possible to determine the location of such potential measuringelectrodes (TPME) that can measure the generated fields inside thevolume covered by the field map. The position of a tracked TPME isdetermined by comparing the measured field value and the modeled fieldvalues. The position in the field map that holds a value matching themeasurement of the tracked TPME is assigned as the location of that PMEReferring to FIG. 8, in some examples, an MEA catheter can be used togenerate a field map which is subsequently used to track the locationsof other electrodes within the organ. Process 280 describes a method fordetermining the position of TPME within an organ using a field mapgenerated using an MEA catheter in which the MEA catheter can be removedprior to tracking the location of the TPME. In step 281, currentinjecting electrodes are positioned in a fixed location in the cavity(the electrodes positioned in the fixed locations are referred to inthis example as SCIE). The SCIE are used both for providing the signalsused in generating the field map and for providing the signals used intracking the locations of TPME using the field map. In step 282, amulti-electrode array catheter (MEA) that includes both the MCIE andMPME is inserted into the cavity. The relative locations of the MCIE andMPME on the MEA are known. In some embodiments, the electrodes of theMEA catheter are bundled into a compact configuration that enables theMEA catheter to be delivered to the heart chamber with minimalobstruction. Once inside the heart chamber, the electrodes of thecatheter are deployed into a specified electrode arrangement relative tothe MEA catheter (e.g., to provide known relative locations of the MCIEand MPME). In order to span the space 3 (or more) separate knownconfigurations of MCIE need to inject current. In step 283, current isinjected using the SCIE. In step 284, the system determines the relativelocations of the MCIE configurations on the MEA catheter and computestheoretical potential fields from the MCIE configurations. In step 285,the MCIE inject current using different MCIE configurations. The 3 pairsof MCIE on the MEA catheter inject the current sequentially, one pair ata time, so that it is possible to trace the source of the measured MPMEsignals to a specific pair. In response to current flow between the pairof selected source/sink electrodes, the MPMEs distributed at multiplelocations on the MEA catheter measure the resultant potential fieldpresent at the those multiple locations. The measured potentials arerecorded. In step 286, the tracking of the electrodes on the MEAcatheter is performed by solving an optimization problem that comparesthe measurement collected by MPME as a result of activation of the pairsof MCIE, to expected computed measurements in a given location. Thelocation that minimizes the difference between the computed and measuredpotentials is assigned as electrode location.

In step 287, location data and field measurements are collected atmultiple field mapping points within the cavity. The location data isdetermined using the fields generated by the MCIE and the fieldmeasurements are determined using the fields generated by the SCIE. Instep 288 a closed surface, S, inside the volume and not including theSCIE is generated. In step 290, the voltage distribution on the surfaceis calculated. Using the calculated voltage distribution, in step 292, afield map is defined as the forward operator from the surface to anypoint inside the volume enclosed by the surface. Once a sufficient fieldmap is generated, in step 294, the MEA catheter can be taken out of thebody. This is advantageous when it is desired to have fewer cathetersinside the body organ for clinical reasons, or when certain trackingfields interfere with other fields In step 300, the voltages on thetracked TPME are measured. In step 302, the position of the TPME isdetermined by solving an optimization problem using the previouslygenerated field map.

Representative System

FIG. 9 shows a schematic diagram of an exemplary embodiment of a system100 to facilitate the tracking of a catheter 110 (or multiple catheters)inside the heart cavity of a patient 101 using the pre-determined modelof the field as described above. The catheter 110 is a moveable catheter110 having multiple spatially distributed electrodes. The catheter(s)are used by a physician 103 to perform various medical procedures,including cardiac mapping and/or treatments such as ablation. Moreparticularly, the catheter(s) can be tracked based on measurements offields by the electrodes using a pre-determined model of the field suchas a field map that provides expected signal measurements of the fieldat various locations within the heart cavity.

System 100 includes an electronics module 140 coupled to processing unit120 for controlling the electrodes on catheter 110 and the CIE in thefixed locations, including a signal generation module for injectingcurrent into the heart cavity through the CIEs and a signal acquisitionmodule for measuring potentials through the PMEs. The electronics module140 can be implemented using analog or digital electronics, or acombination of both. Such exemplary configurations, which are intendedto be non-limiting, are now described.

Referring to FIG. 10, the signal generation and acquisition modules areimplemented using analog hardware. The signal generation module (SGM)depicted supports 8 CIEs defining 4 source/sink electrode pairs, whereSRC refers to a source electrode and SNK refers to a sink electrode. Itshould be appreciated that other configurations of CIE are possible.Examples of such a configuration could be quadruples involving 4 CIE, ornon-symmetrical configurations involving 3 CIE. For simplicity themethod using electrode pairs will be explained, but the same method canbe applied using other configurations. In general, at least 3 separateconfigurations of CIE are used in order to span the 3D space and provideXYZ coordinates of other electrodes. For the purpose of this example,each pair is driven using a 5 kHz oscillating 1 mA current source. Otherdriving frequencies, for example, 10 kHz, can be used. A selector switchis used to select each of the pairs sequentially based on controlsignals provided by the processing unit or other control logic. Eachchannel in the signal generation module is connected to a currentinjecting electrode. In this case the source and sink electrodes arepre-selected permanently such that each electrode is always either asource or a sink, although this need not be the case in otherembodiments

The signal acquisition module (SAM) buffers and amplifies the signals asthey are collected by the potential measuring electrodes. The bufferprevents the acquisition system from loading the signals collected bythe electrodes. After buffering and amplification, the signals are splitand filtered into two channels, one for detecting the tracking signal(i.e., the signals produced in response to the CIEs) and one fordetecting the signal generated by the heart's electrical activation(i.e., cardiac mapping). Because the heart's electrical activity (e.g.,the cardiac signals) is primarily below 2 kHz, a low pass filter (LPF)is used to separate the cardiac mapping potential signals from thoseproduced in response to the CIEs. The low pass filter may be implementedas an active filter responsible for both filtering and amplification.The signal is then sampled by an analog to digital converter. To supportbandwidth and resolution requirements the converter may sample at >4 kHzat 15 bits per sample. After sampling, the signals are passed to theprocessing unit for further analysis. Both the LPF and A/D may beconfigured such that the filter and sample frequency can be changed bysoftware control (not drawn).

The second channel following the input buffer detects the trackingsignal (e.g., the signals measured in response to current injected bythe CIE). In this embodiment, the detection is implemented using alock-in amplifier approach to detect amplitude. It should be appreciatedthat other implementation can be used to accomplish the same task. Inthis channel the signal is first filtered using a band pass filter (BPF)whose pass band frequency is centered on the 5 kHz generated by the SGM.Following the BPF, the signal is multiplied by the same 5 kHz signalgenerated by the SGM using a mixer. As a result, the signal is downconverted to DC such that its value following the down conversion isproportional to its amplitude before the down-conversion. The signal isthen filtered using a very narrow LPF of roughly 100 Hz. The filterbandwidth has two effects. On the one hand, the narrower the filter thebetter noise performance will be. On the other hand, the wider thefilter, the more tracking updates are available per second. A filtersetting of 100 Hz provides excellent noise performance. After filtering,the signal is amplified and sampled by an analog to digital converter.The converter in this case may sample at 200 Hz using 15 bits persample. After sampling, the signals are passed to the processing unitfor further analysis. As before, the channel properties can beconfigured to be changed by software control (not drawn).

While the embodiment described above in relation to FIG. 10 described ananalog signal generation and acquisition modules, in some examples adigital implementation can be used. For example, referring to FIG. 11,the signal generation and acquisition modules have a digitalimplementation. The SGM generates the required signals using an array ofn digital to analog converters (D/A). In a preferred embodiment n=8. Inanother preferred embodiment, n=6. It should be appreciated that insteadof n D/As it is possible to use fewer D/As and a multiplexed sample andhold amplifier. The signals generated by the D/As are controlled andtimed by the processing unit. In one embodiment, the signals may mimicthose described in the analog implementation whereby a sinusoidal signalis switched between electrodes. In other embodiments, however, thedigital implementation provides more flexibility in that more complexsignals (e.g., different frequencies, simultaneous activation ofmultiple electrodes) may be driven. After the conversion to an analogsignal, the signals are buffered by an amplifier capable of driving thenecessary current (<2 mA) at relevant frequencies (<30 kHz). Afterbuffering, a processor controlled switch is used to support a highimpedance mode. This is necessary in order to block a particularelectrode from acting as a source or a sink at a particular time.

In the SAM hardware, an input stage amplifies and buffers the signal.Following amplification the signal is low pass filtered in a wide enoughband such that both the heart's electrical activity (<2 kHz) and signalsgenerated by the SGM are kept inside the filtered band. In FIG. 11 thefrequency band is 15 kHz. Following the filter, the signal is sampledabove Nyquist frequency (>30 kHz) at 15 bits per sample. The sampledsignals are then transferred to the processing unit which uses digitalsignal processing (DSP) techniques to filter the two channels in eachelectrode and down-convert the tracking signal appropriately.

A relatively small number of CIEs can result in a relatively largenumber of possible electrode pair combinations that can be activated toenable different potential field configurations to be formed inside theheart cavity, in which the catheter 110 is deployed and thus enhance therobustness of the tracking procedure. For example, six (6) electrodescan be paired into fifteen (15) combinations of different source/sinkpairs, thus resulting in fifteen different potential fields, for aparticular potential value, formed inside the medium. As noted above, toachieve high robustness of the tracking procedure, the varioussource/sink electrodes may be mounted at different locations in theorgan or relative to the organ.

Preferably, the current injecting electrodes are designed to have lowimpedance at the interface between electrode and blood. The impedancebetween electrodes and blood is determined by the surface area of theelectrode and electrode material. The larger the surface area, the lowerthe impedance In some embodiments, a larger surface area for CIEs ispreferred in order to reduce their impedance at the interface to bloodand allow the injection of current. In some embodiments, specializedcoatings such as Platinum Black, Iridium Oxide and Titanium Nitride maybe applied to one or more of the CIEs, one or more of the PMEs, or allof the catheter electrodes to reduce impedance of electrodes for a givensurface area.

In yet further embodiments, one or more of the electrodes can be drivento function as both a CIE and a PME. For example, when it is desired touse an electrode as both PME and CIE, the electrode is connected to botha signal acquisition module and a signal generation module. For example,when the electrode is not used as a CIE to drive a current, the switchin the signal generation module corresponding to the respectiveelectrode is opened. Accordingly, time division multiplexing schemes inthe driving electronics of module can be used to operate a givencatheter electrode as either a CIE or a PME. In yet another example, theelectronics module can drive a given electrode so that it functions as aCIE at high frequencies and a PME at low frequencies (such as might beuseful for cardiac mapping.)

As noted above, the PMEs on catheter 110 can also used for cardiacmapping, such as that described in commonly owned patent applicationSer. No. 11/451,898, entitled “NON-CONTACT CARDIAC MAPPING, INCLUDINGMOVING CATHETER AND MULTI-BEAT INTEGRATION” and filed Jun. 13, 2006, thecontents of which are incorporated herein by reference. As also notedabove, because the frequency of the current injected by CIEs (e.g., 5kHz) is much higher than the frequency of the electrical activity of thepatient's heart (e.g., the frequency of the cardiac signals), the signalacquisition module can separate signals measured by the PMEs based onfrequency to distinguish tracking signals measured in response tocurrents injected by the CIE from cardiac mapping signals (e.g.,frequencies higher than 2 kHz, and lower than 2 kHz, respectively.)Furthermore, in additional embodiments, catheter 110 may includeseparate electrodes used only for cardiac mapping.

The system 100 further includes the processing unit 120 which performsseveral of the operations pertaining to the tracking procedure,including the determination of catheter electrode locations that resultin the best fit between the measured signals and those calculated fordifferent positions of the catheter. Additionally, the processing unit120 can subsequently also perform the cardiac mapping procedure,including a reconstruction procedure to determine the physiologicalinformation at the endocardium surface from measured signals, and mayalso perform post-processing operations on the reconstructedphysiological information to extract and display useful features of theinformation to the operator of the system 100 and/or other persons(e.g., a physician). For example, the system 100 can display thelocation of the catheter(s) relative to a surface of the heart. In someembodiments, a stabilized representation of the heart and position canbe used to display the position of the catheter as the shape of theheart changes during the heart's cycle.

The signals acquired by the various electrodes of catheter 110 duringthe tracking and/or the mapping procedure are passed to the processingunit 120 via electronics module 140. As described above, electronicsmodule 140 can be used to amplify, filter and continuously sampleintracardiac potentials measured by each electrode.

In some embodiments, the electronics module 140 is implemented by use ofintegrated components on a dedicated printed circuit board. In otherembodiments, some of the signal conditioning tasks may be implemented ona CPU, FPGA or DSP after sampling. To accommodate safety regulations,the signal conditioning module is isolated from high voltage powersupplies. The electronics module is also protected from defibrillationshock, and interference caused by nearby pacing or ablation.

The processing unit 120 shown in FIG. 9 is a processor-based device thatincludes a computer and/or other types of processor-based devicessuitable for multiple applications. Such devices can include volatileand non-volatile memory elements, and peripheral devices to enableinput/output functionality. Such peripheral devices include, forexample, a CD-ROM drive and/or floppy drive, or a network connection,for downloading related content to the connected system. Such peripheraldevices may also be used for downloading software containing computerinstructions to enable general operation of the respective unit/module,and for downloading software implemented programs to perform operationsin the manner that will be described in more detailed below with respectto the various systems and devices shown in FIG. 9. Alternatively, thevarious units/modules may be implemented on a single or multiprocessor-based platform capable of performing the functions of theseunits/modules. Additionally or alternatively, one or more of theprocedures performed by the processing unit 120 and/or electronicsmodule 140 may be implemented using processing hardware such as digitalsignal processors (DSP), field programmable gate arrays (FPGA),mixed-signal integrated circuits, ASICS, etc. The electronics module 140is typically implemented using analog hardware augmented with signalprocessing capabilities provided by DSP, CPU and FPGA devices.

As additionally shown in FIG. 9, the system 100 includes peripheraldevices such as printer 150 and/or display device 170, both of which areinterconnected to the processing unit 120. Additionally, the system 100includes storage device 160 that is used to store data acquired by thevarious interconnected modules, including the volumetric images, rawdata measured by electrodes and the resultant endocardium representationcomputed there from, the reconstructed physiological informationcorresponding to the endocardium surface, etc.

Other Embodiments

While in at least some of the embodiments described above, the CIE pairsare shown as positioned in the coronary sinus, other locations could beused to secure the CIE pairs. For example, the CIE pairs could besecured in the, atrial appendage, apex, and the like. Additionally, insome embodiments, the CIE pairs could be positioned outside of thepatient's body (e.g., affixed to the surface of the patient's chest). Insome additional embodiments, the CIE pairs could be positioned withinthe organ itself, e.g., within the heart chamber.

The methods and systems described herein are not limited to a particularhardware or software configuration, and may find applicability in manycomputing or processing environments. The methods and systems can beimplemented in hardware, or a combination of hardware and software,and/or can be implemented from commercially available modulesapplications and devices. Where the implementation of the systems andmethods described herein is at least partly based on use ofmicroprocessors, the methods and systems can be implemented in one ormore computer programs, where a computer program can be understood toinclude one or more processor executable instructions. The computerprogram(s) can execute on one or more programmable processors, and canbe stored on one or more storage medium readable by the processor(including volatile and non-volatile memory and/or storage elements),one or more input devices, and/or one or more output devices. Theprocessor thus can access one or more input devices to obtain inputdata, and can access one or more output devices to communicate outputdata. The input and/or output devices can include one or more of thefollowing: Random Access Memory (RAM), Redundant Array of IndependentDisks (RAID), floppy drive, CD, DVD, magnetic disk, internal hard drive,external hard drive, memory stick, or other storage device capable ofbeing accessed by a processor as provided herein, where suchaforementioned examples are not exhaustive, and are for illustration andnot limitation.

The computer program(s) can be implemented using one or more high levelprocedural or object-oriented programming languages to communicate witha computer system; however, the program(s) can be implemented inassembly or machine language, if desired. The language can be compiledor interpreted. The device(s) or computer systems that integrate withthe processor(s) can include, for example, a personal computer(s),workstation (e.g., Sun, HP), personal digital assistant (PDA), handhelddevice such as cellular telephone, laptop, handheld, or another devicecapable of being integrated with a processor(s) that can operate asprovided herein. Accordingly, the devices provided herein are notexhaustive and are provided for illustration and not limitation.

References to “a microprocessor” and “a processor”, or “themicroprocessor” and “the processor,” can be understood to include one ormore microprocessors that can communicate in a stand-alone and/or adistributed environment(s), and can thus be configured to communicatevia wired or wireless communications with other processors, where suchone or more processor can be configured to operate on one or moreprocessor-controlled devices that can be similar or different devices.Furthermore, references to memory, unless otherwise specified, caninclude one or more processor-readable and accessible memory elementsand/or components that can be internal to the processor-controlleddevice, external to the processor-controlled device, and can be accessedvia a wired or wireless network using a variety of communicationsprotocols, and unless otherwise specified, can be arranged to include acombination of external and internal memory devices, where such memorycan be contiguous and/or partitioned based on the application.Accordingly, references to a database can be understood to include oneor more memory associations, where such references can includecommercially available database products (e.g., SQL, Informix, Oracle)and also proprietary databases, and may also include other structuresfor associating memory such as links, queues, graphs, trees, with suchstructures provided for illustration and not limitation.

Accordingly, other embodiments are within the scope of the followingclaims.

1. A method comprising: securing multiple sets of current injectingelectrodes to an organ in a patient's body; causing current to flowamong the multiple sets of current injecting electrodes to generate afield in the organ; in response to current flow caused by the multiplesets of current injecting electrodes, measuring the field at each of oneor more additional electrodes, determining expected signal measurementsof the field inside the organ using a pre-determined model of the field;and determining a position of each of the one or more additionalelectrodes in the organ based on the measurements made by the additionalelectrodes and the determined expected signal measurements of the field.2. The method of 1, wherein determining the position of the one or moreadditional electrodes in the organ based on measurements made by theadditional electrodes and the determined expected signal measurements ofthe field comprises solving an optimization problem that minimizes acollective difference between each of the measured signals and anestimate for each of the respective measured signals as a function ofthe position of the measurement.
 3. The method of 2, wherein theestimate for each of the respective measured signals comprises adifferentiable function.
 4. The method of 1, wherein the one or moreadditional electrodes comprise one or more electrodes used fordelivering ablation energy for ablating tissue of the organ.
 5. Themethod of 1, wherein the one or more additional electrodes comprise oneor more electrodes used for measuring the electrical activity of theorgan.
 6. The method of claim 1, further comprising generating thepre-determined model of the field.
 7. The method of claim 6, whereingenerating the pre-determined model of the field comprises: causingcurrent to flow among the multiple sets of current injecting electrodesto generate a field in an organ; obtaining the positions of one or moremeasuring electrodes; in response to the current flow, measuring thefield at multiple locations in the organ using the one or more measuringelectrodes; modeling the field using the measurements of the fieldmeasured by the one or more measuring electrodes and the positions ofthe one or more measuring electrodes.
 8. The method of 7, whereinmodeling the field comprises modeling the field based on physicalcharacteristics.
 9. The method of 8, wherein modeling the field based onphysical characteristics comprises using Laplace's equation.
 10. Themethod of 8, wherein modeling the field based on physicalcharacteristics comprises using Poisson's equation.
 11. The method of 8,wherein modeling the field based on physical characteristics comprisesmodeling a homogeneous medium.
 12. The method of 8, wherein modeling thefield based on physical characteristics comprises modeling aninhomogeneous medium.
 13. The method of 7, wherein modeling of the fieldfurther comprises representing the model using a function thatcorrelates field measurements with position coordinates.
 14. The methodof 1, wherein the pre-determined model of the field comprises a fieldmap.
 15. The method of 14, wherein the field map is a function thatcorrelates the expected signal measurements with position coordinateswithin the organ.
 16. The method of 15, wherein the function is adifferentiable function.
 17. The method of 1, wherein measuring thefield comprises measuring potentials.
 18. The method of claim 1, whereinthe current-injecting electrodes operate at a frequency different fromthe frequency of normal electrical activity in the organ.
 19. The methodof claim 1, wherein the organ is a patient's heart.
 20. A systemcomprising: multiple sets of current injecting electrodes configured tobe secured to an organ in a patient's body; one or more additionalelectrodes configured to be positioned within the organ in the patient'sbody; an electronic control system coupled to the multiple sets ofcurrent injecting electrodes and the one or more additional electrodes,the electronic control system being configured to: cause current to flowamong the multiple sets of current injecting electrodes to generate afield in the organ; in response to current flow caused by the multiplesets of current injecting electrodes, measure the field at each of oneor more additional electrodes, a processing system coupled to theelectronic system and configured to: determine expected signalmeasurements of the field inside the organ using a pre-determined modelof the field; and determine a position of each of the one or moreadditional electrodes in the organ based on the measurements made by theadditional electrodes and the determined expected signal measurements ofthe field.
 21. The system of 20, wherein the processing system isconfigured to solve an optimization problem that minimizes a collectivedifference between each of the measured signals and an estimate for eachof the respective measured signals as a function of the position of themeasurement.
 22. The system of 20, wherein the one or more additionalelectrodes comprise one or more electrodes used for delivering ablationenergy for ablating tissue of the organ.
 23. The system of 20, whereinthe one or more additional electrodes comprise one or more electrodesused for measuring the electrical activity of the organ.
 24. The systemof claim 20, wherein the processing system is further configured togenerate the pre-determined model of the field.
 25. A method comprising:securing at least three sets of current injecting electrodes to an organin a patient's body; causing current to flow among the multiple sets ofcurrent injecting electrodes to generate a field in the organ; using amulti-electrode array located on a multi-electrode array catheter in theorgan for tracking a position of the multi-electrode array catheterrelative to the current injecting electrodes; measuring the fieldgenerated by the current injecting electrodes in multiple locations inthe organ using the multi-electrode array; modeling the field using themeasurements and the positions; determining expected signal measurementsof the field at additional locations within the organ based on the modelof the field; and determining a position of one or more additionalelectrodes in the organ relative to the current injecting electrodesbased on measurements made by the additional electrodes and thedetermined expected signal measurements of the field.
 26. The method of25, further comprising removing multi-electrode array catheter from theorgan prior to determining the position of one or more additionalelectrodes in the organ.
 27. The method of 25, wherein the one or moreadditional electrodes comprise one or more electrodes mounted on one ofmore additional catheters.
 28. The method of 25, wherein the one or moreadditional electrodes comprise one or more electrodes of themulti-electrode array.
 29. The method of 25, wherein modeling the fieldbased on physical characteristics comprises using Laplace's equation.30. The method of 25, wherein modeling the field comprises modeling thefield based on physical characteristics.
 31. The method of 30, whereinmodeling the field based on physical characteristics comprises usingPoisson's equation.
 32. The method of 30, wherein modeling the fieldbased on physical characteristics comprises modeling a homogeneousmedium.
 33. The method of 30, wherein modeling the field based onphysical characteristics comprises modeling an inhomogeneous medium. 34.The method of 30, wherein modeling of the field further comprisesrepresenting the model using a function that correlates fieldmeasurements with position coordinates.
 35. The method of 25, whereinthe additional locations within the organ comprise positions within theorgan where the field was not measured.
 36. The method of 35, whereinthe positions within the organ where field was not measured comprisepositions that are more than 5 mm away from positions where the fieldwas measured.
 37. The method of 35, wherein the positions within theorgan where field was not measured comprise positions that are not lyingbetween positions where the field was measured.
 38. The method of 37,wherein determining a position of one or more additional electrodes inthe field based on measurements made by the additional electrodes andthe determined expected signal measurements of the field comprisessolving an optimization problem that minimizes collective differencebetween each of the measured signals and an estimate for each of therespective measured signals as a function of the position of themeasurement.
 39. The method of 25, wherein determining expected signalmeasurements comprises determining expected signal measurements using anon-interpolation based calculation.
 40. The method of claim 25, whereinthe measuring of the field at the multiple locations comprises moving acatheter having one or more measuring electrodes to multiple locationswithin the organ, and using the measuring electrodes to measure thefield for each of the multiple locations of the catheter.
 41. The methodof claim 40, wherein the additional locations correspond to regionsinside the organ not interrogated by the movement of the catheter. 42.The method of claim 25, wherein the multiple sets of current-injectingelectrodes comprise at least three sets of current injecting electrodes,and wherein the causing of the current flow comprises causing current toflow between each set of current injecting electrodes, and wherein thefield measured in response to the current flow comprise a fieldmeasurement for each set of the current injecting electrodes for each ofthe multiple positions.
 43. The method of claim 25, wherein modeling thefield comprises generating a field map.
 44. The method of claim 25,further comprising displaying the determined location of the measuringelectrode relative to a surface of the organ.
 45. A system comprising:at least three sets of current injecting electrodes configured to besecured to an organ in a patient's body; a multi-electrode arraycatheter comprising a multi-electrode array configured to be inserted inthe organ for tracking a position of the multi-electrode array catheterrelative to the current injecting electrodes; one or more additionalelectrodes configured to be inserted in the organ; an electronic controlsystem coupled to the at least three sets of current injectingelectrodes, to the multi-electrode array catheter, and to the one ormore additional electrodes, the electronic control system beingconfigured to: cause current to flow among the multiple sets of currentinjecting electrodes to generate a field in the organ; measure the fieldgenerated by the current injecting electrodes in multiple locations inthe organ using the multi-electrode array; a processing system coupledto the electronic system and configured to: model the field using themeasurements and the positions; determine expected signal measurementsof the field at additional locations within the organ based on the modelof the field; and determine a position of the one or more additionalelectrodes in the organ relative to the current injecting electrodesbased on measurements made by the additional electrodes and thedetermined expected signal measurements of the field.
 46. The system of45, wherein the one or more additional electrodes comprise one or moreelectrodes mounted on one of more additional catheters.
 47. The systemof 45, wherein the one or more additional electrodes comprise one ormore electrodes of the multi-electrode array.
 48. The system of claim45, wherein the multiple sets of current-injecting electrodes compriseat least three sets of current injecting electrodes.